using Server-sent-events (SSE) for notifications support (and background processing)

WIP.

backend/pkg/constants.go

@@ -9,6 +9,9 @@ const (
 	ContextKeyTypeDatabase string = "REPOSITORY"
 	ContextKeyTypeLogger   string = "LOGGER"
 
+	ContextKeyTypeSSEServer        string = "SSE_SERVER"
+	ContextKeyTypeSSEClientChannel string = "SSE_CLIENT_CHANNEL"
+
 	ContextKeyTypeAuthUsername string = "AUTH_USERNAME"
 	ContextKeyTypeAuthToken    string = "AUTH_TOKEN"
 

backend/pkg/web/handler/server_sent_event.go

@@ -0,0 +1,36 @@
+package handler
+
+import (
+	"github.com/fastenhealth/fasten-onprem/backend/pkg"
+	"github.com/fastenhealth/fasten-onprem/backend/pkg/web/middleware"
+	"github.com/gin-gonic/gin"
+	"io"
+)
+
+// SSEStream is a handler for the server sent event stream (notifications from background processes)
+// see: https://github.com/gin-gonic/examples/blob/master/server-sent-event/main.go
+// see: https://stackoverflow.com/questions/66327142/selectively-send-event-to-particular-clients
+//
+// test using:
+// curl -N -H "Authorization: Bearer xxxxx" http://localhost:9090/api/secure/sse/stream
+func SSEStream(c *gin.Context) {
+
+	//logger := c.MustGet(pkg.ContextKeyTypeLogger).(*logrus.Entry)
+	//databaseRepo := c.MustGet(pkg.ContextKeyTypeDatabase).(database.DatabaseRepository)
+	v, ok := c.Get(pkg.ContextKeyTypeSSEClientChannel)
+	if !ok {
+		return
+	}
+	clientChan, ok := v.(middleware.ClientChan)
+	if !ok {
+		return
+	}
+	c.Stream(func(w io.Writer) bool {
+		// Stream message to client from message channel
+		if msg, ok := <-clientChan; ok {
+			c.SSEvent("message", msg)
+			return true
+		}
+		return false
+	})
+}

backend/pkg/web/middleware/server_sent_event.go

@@ -0,0 +1,120 @@
+package middleware
+
+import (
+	"fmt"
+	"github.com/fastenhealth/fasten-onprem/backend/pkg"
+	"github.com/gin-gonic/gin"
+	"log"
+	"time"
+)
+
+func SSEHeaderMiddleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Writer.Header().Set("Content-Type", "text/event-stream")
+		c.Writer.Header().Set("Cache-Control", "no-cache")
+		c.Writer.Header().Set("Connection", "keep-alive")
+		c.Writer.Header().Set("Transfer-Encoding", "chunked")
+		c.Next()
+	}
+}
+
+func SSEServerMiddleware() gin.HandlerFunc {
+
+	// Initialize new streaming server
+	stream := NewSSEServer()
+
+	///TODO: testing only
+	go func() {
+		for {
+			time.Sleep(time.Second * 10)
+			now := time.Now().Format("2006-01-02 15:04:05")
+			currentTime := fmt.Sprintf("The Current Time Is %v", now)
+
+			// Send current time to clients message channel
+			stream.Message <- currentTime
+		}
+	}()
+
+	return func(c *gin.Context) {
+		// Initialize client channel
+		clientChan := make(ClientChan)
+
+		// Send new connection to event server
+		stream.NewClients <- clientChan
+
+		defer func() {
+			// Send closed connection to event server
+			stream.ClosedClients <- clientChan
+		}()
+
+		c.Set(pkg.ContextKeyTypeSSEServer, stream)
+		c.Set(pkg.ContextKeyTypeSSEClientChannel, clientChan)
+
+		c.Next()
+	}
+}
+
+//####################################################################################################
+//TODO: this should all be moved into its own package
+//####################################################################################################
+
+// New event messages are broadcast to all registered client connection channels
+// TODO: change this to be use specific channels.
+type ClientChan chan string
+
+// Initialize event and Start procnteessing requests
+// this should be a singleton, to ensure that we're always broadcasting to the same clients
+// see: https://refactoring.guru/design-patterns/singleton/go/example
+func NewSSEServer() (event *SSEvent) {
+	event = &SSEvent{
+		Message:       make(chan string),
+		NewClients:    make(chan chan string),
+		ClosedClients: make(chan chan string),
+		TotalClients:  make(map[chan string]bool),
+	}
+
+	go event.listen()
+
+	return
+}
+
+// It keeps a list of clients those are currently attached
+// and broadcasting events to those clients.
+type SSEvent struct {
+	// Events are pushed to this channel by the main events-gathering routine
+	Message chan string
+
+	// New client connections
+	NewClients chan chan string
+
+	// Closed client connections
+	ClosedClients chan chan string
+
+	// Total client connections
+	TotalClients map[chan string]bool
+}
+
+// It Listens all incoming requests from clients.
+// Handles addition and removal of clients and broadcast messages to clients.
+func (stream *SSEvent) listen() {
+	for {
+		select {
+		// Add new available client
+		case client := <-stream.NewClients:
+			stream.TotalClients[client] = true
+			log.Printf("Client added. %d registered clients", len(stream.TotalClients))
+
+		// Remove closed client
+		case client := <-stream.ClosedClients:
+			delete(stream.TotalClients, client)
+			close(client)
+			log.Printf("Removed client. %d registered clients", len(stream.TotalClients))
+
+		// Broadcast message to client
+		case eventMsg := <-stream.Message:
+			for clientMessageChan := range stream.TotalClients {
+				clientMessageChan <- eventMsg
+			}
+		}
+	}
+}

backend/pkg/web/server.go

@@ -70,6 +70,8 @@ func (ae *AppEngine) Setup() (*gin.RouterGroup, *gin.Engine) {
 
 				secure.POST("/query", handler.QueryResourceFhir)
 
+				//server-side-events handler
+				secure.GET("/sse/stream", middleware.SSEHeaderMiddleware(), middleware.SSEServerMiddleware(), handler.SSEStream)
 			}
 
 			if ae.Config.GetBool("web.allow_unsafe_endpoints") {